The present invention relates to hydrogen/oxygen fuel cells; more particularly, to stacks comprising a plurality of individual cells connected by interconnect elements; and most particularly, to a fuel cell assembly wherein internal temperature, especially temperature of the cell itself, is modulated by periodic reversal of the direction of air flow across the cathode.
Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are deposited on opposite surfaces of a permeable electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a xe2x80x9csolid oxide fuel cellxe2x80x9d (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode where it is ionized. The oxygen ions diffuse through the electrolyte and combine with hydrogen ions to form water. The cathode and the anode are connected externally through the load to complete the circuit whereby electrons are transferred from the anode to the cathode. When hydrogen is derived from xe2x80x9creformedxe2x80x9d hydrocarbons, the reformate gas includes CO which is converted to CO2 at the anode. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.
An SOFC operates at a temperature, typically, of about 750xc2x0 C. or higher. The reaction is exothermic, so the SOFC requires active cooling during operation, typically by flowing cooler air across the cathode. Conversely, at startup from ambient temperatures, the SOFC requires heating for the catalytic electrolyte to begin ionizing oxygen, typically by flowing heated air across the cathode.
A serious problem arises in thermal management within an SOFC. Because the cathode is highly vulnerable to cracking and consequent failure from thermal stresses, temperature differences greater than about 200xc2x0 C. are unacceptable. Air flows through a fuel cell from introduction at an upstream edge of the cathode to discharge across a downstream edge, undergoing temperature change during such flow. Thus, the cathode experiences an inherent temperature difference between the upstream and downstream edges, and between itself and the temperature-modulating air. Since the permissible temperature difference (xcex94T) between the temperature of the heating air and the internal temperature of the SOFC is limited, long warmup times on the order of several hours typically are required, whereas for automotive uses, startup times of about ten minutes or less are highly desirable.
Similarly, large volumes of cooling air are required during operation because the permissible xcex94T for cooling is limited. Providing such large volumes is parasitically consumptive of power being generated by the fuel cell, thereby reducing the net power output thereof, since it requires a relatively large blower having a relatively large electric motor.
What is needed is a means for providing a higher difference between the average temperature of cathode entry air and the average temperature of cathode exit air for heating and cooling a fuel cell cathode to shorten the startup time and to reduce the volume of cooling air required.
It is a principal object of the present invention to provide an improved thermal management method and apparatus for an SOFC wherein startup may be achieved in a short period of time.
It is a further object of the invention to provide such a method and apparatus wherein lower volumes of cooling air are required.
Briefly described, a fuel cell assembly in accordance with the invention has means for providing tempered air to, and removing spent air from, air-flow passages across the cathode(s). The air flow path includes means for reversing the direction of flow across the cathode(s) periodically to reverse the roles of the leading and trailing edges of the cathode(s) to prevent temperature differences across the cathodes(s) from exceeding 200xc2x0 C., and thus to prevent damage to the cathode(s) from thermally-induced stresses during startup heating and steady-state cooling.